Turbochargers are well known devices for supplying air to the intake of an internal combustion engine at pressures above atmospheric (boost pressures). A conventional turbocharger essentially comprises an exhaust gas driven turbine wheel mounted on a rotatable shaft within a turbine housing. Rotation of the turbine wheel rotates a compressor wheel mounted on the other end of the shaft within a compressor housing. The compressor wheel delivers compressed air to the engine intake manifold. The turbocharger shaft is conventionally supported by journal and thrust bearings, including appropriate lubricating systems, located within a central bearing housing connected between the turbine and compressor wheel housings.
In known turbochargers, the turbine stage comprises a turbine chamber within which the turbine wheel is mounted, an annular inlet passageway defined between facing radial walls arranged around the turbine chamber, an inlet arranged around the inlet passageway, and an outlet passageway extending from the turbine chamber. The passageways and chambers communicate such that pressurised exhaust gas admitted to the inlet chamber flows through the inlet passageway to the outlet passageway via the turbine and rotates the turbine wheel. It is also well known to trim turbine performance by providing vanes, referred to as nozzle vanes, in the inlet passageway so as to deflect gas flowing through the inlet passageway towards the direction of rotation of the turbine wheel.
Turbines may be of a fixed or variable geometry type. Variable geometry turbines differ from fixed geometry turbines in that the size of the inlet passageway can be varied to optimise gas flow velocities over a range of mass flow rates so that the power output of the turbine can be varied to suite varying engine demands. For instance, when the volume of exhaust gas being delivered to the turbine is relatively low, the velocity of the gas reaching the turbine wheel is maintained at a level which ensures efficient turbine operation by reducing the size of the annular inlet passageway.
In one known type of variable geometry turbine, one wall of the inlet passageway is defined by an axially moveable wall member, generally referred to as a nozzle ring. The position of the nozzle ring relative to a facing wall of the inlet passageway is adjustable to control the axial width of the inlet passageway. Thus, for example, as gas flowing through the turbine decreases the inlet passageway width may also be decreased to maintain gas velocity and optimise turbine output. Such nozzle rings essentially comprise a radially extending wall and inner and outer axially extending annular flanges. The annular flanges extend into an annular cavity defined in the turbine housing (a part of the housing which in practice be provided by the bearing housing) which accommodates axial movement of the nozzle ring.
The nozzle ring may be provided with vanes which extend into the inlet passageway and through slots provided on the facing wall of the inlet passageway to accommodate movement of the nozzle ring. Alternatively, vanes may extend from the fixed wall through slots provided in the nozzle ring. Generally the nozzle ring is supported on rods extending parallel to the axis of rotation of the turbine wheel and is moved by an actuator which axially displaces the rods. Various forms of actuator are known for use in variable geometry turbines, including pneumatic, hydraulic and electric actuators, mounted externally of the turbocharger and connected to the variable geometry system via appropriate linkages.
In addition to the conventional control of a variable geometry turbine to optimise turbocharger performance, it is also known to take advantage of the facility to minimise the turbocharger inlet to provide an exhaust braking function. Exhaust brake systems of various forms are widely fitted to vehicle engine systems, in particular to compression ignition engines (diesel engines) used to power large vehicles such as trucks. Conventional exhaust brake systems comprise a valve in the exhaust line from the engine which when activated substantially blocks the engine exhaust (fully blocking he exhaust line would stall the engine). This creates back pressure which retards rotation of the engine providing a braking force which is transmitted to the vehicle wheels through the vehicle drive train. The exhaust braking may be employed to enhance the effect of the conventional friction brakes acting on the vehicle wheels, or in some circumstances be used independently of the normal wheel braking system, for instance to control down hill speed of a vehicle. With some exhaust brake systems the brake is set to activate automatically when the engine throttle is closed (i.e. when the driver lifts his foot from the throttle pedal), and in others the exhaust brake may require manual activation by the driver, such as depression of a separate brake pedal. The exhaust brake valve is generally controllable to modulate the braking effect, for example to maintain a constant vehicle speed.
With a variable geometry turbine it is not necessary to provide a separate exhaust brake valve. Rather, the turbine inlet passageway may simply be closed to its minimum flow area when braking is required. The level of braking may be modulated by control of the inlet passageway size by appropriate control of the axial position of the nozzle ring (or other variable geometry mechanism). Whilst having the advantage of obviating the need to provide a separate exhaust brake valve, there are however problems associated with operation of variable geometry turbines in an exhaust braking mode.
In particular with the modem highly efficient turbines, a relatively high air flow is still delivered to the engine as the inlet passageway is reduced towards the minimum width. This can result in engine cylinder pressures approaching or exceeding acceptable limits if the inlet passage is closed too far. Accordingly there is a practical limit on the extent to which the inlet passage can be closed in braking mode, which in turn limits the effective braking force that can be provided by control of a conventional variable geometry turbine.
It is an object of the present invention to obviate or mitigate the above disadvantage.